I was unable to find 'open city'. But was able to find civilian:
" A civilian is any person who does not belong to any of the following categories: members of the armed forces, militias or volunteer corps, organized resistance movements, and residents of an occupied territory who spontaneously take up arms. If there is any doubt whether a person is civilian, then he or she is to be considered a civilian. (Protocol I, Art. 50, Sec. 1)"

I do not see mention of workers. This of course does not preclude it being mentioned elsewhere.

But this still does not matter. If you are at war then you should commit yourself to total war. If this means that terror by way of bombing will shorten the war then by all means bomb away.